A large valve，usually installed above the ram preventers，that forms a seal in the annular space between the pipe and wellbore or，if no pipe is present，on the wellbore itself is called the（）.

A.flame arrestor
B.flame screen
C.detonation arrestor
D.detonation blocker

A.slipway
B.graving dock
C.ground warp
D.caisson dock

A.a 2-second blast every 20 seconds
B.a 4-second blast every 20 seconds
C.an 8-second blast every 30 seconds
D.a 10-second blast every 30 seconds